Method of constructing a gaseous laser

ABSTRACT

A method of making a gaseous laser is disclosed using a thin-walled, ceramic tube such as alumina. Heat conducting members are assembled and aligned in a spaced-apart relationship within the tube. Coaxially aligned cylindrical shields are affixed to the heat-conducting members to control gas pumping within the tube. Bore-defining aperatures in the heat-conducting members, or in sputter-resistant discs affixed to the heat-conducting members, are aligned by a mandrel under tension at which time the parts are permanently bonded within the tube.

BACKGROUND OF THE INVENTION

The present invention relates to gas lasers and in particular to gaseousion lasers having an improved laser discharge tube.

There are presently several varieties of commercially-available gas ionlasers. One type has a discharge tube which uses a plurality of graphitediscs within a gas-confining glass tube. See for example U.S. Pat. No.3,619,810. In another type, thick-walled beryllia (BeO or berylliumoxide) segments are bonded together to make a discharge bore. See forexample U.S. Pat. No. 3,760,213.

Another type of discharge tube is described in U.S. Pat. No. 3,501,714.In this patent a thin-walled, precision, ceramic tube is used. One ofthe suggested ceramic materials is alumina (Al₂ O₃ or aluminum oxide).Heat generated in the discharge is conducted out of the tube through aseries of closely-spaced cylindrical sections which expand into contactwith the ceramic tube when they are heated during operation of the tube.

This design has a number of advantages over the thick-walled BeO typedischarge tube. In a BeO capillary tube with an inner bore diameter ofabout two millimeters and outer diameter of the order of one toone-and-a-half centimeters, the thermal heat flow through the berylliumoxide produces a tensile stress in the outer wall of the tube, which ina typical ion laser amounts to ten to fifteen percent of the breakingstrength of the ceramic. Since alumina has about one seventh the thermalconductivity of beryllium oxide and since the stress is inverselyproportional to thermal conductivity, the stress in an alumina capillarytube of the same dimensions could be sufficient to break the tube.

However, in a relatively thin walled tube (neglecting end effects) thecircumferential and longitudinal stress tension in the outer layers isgiven by: (1/2) Δtαε/(1-ν); where Δt is the temperature gradiant acrossthe tube wall; α is the linear coefficient of expansion of the material,ε is Young's modulus for the material; and ν is Poisson's ratio. If thealumina tube is made on the order of 11/2 inches in diameter, the areaavailable for heat flow substantially reduces Δt so that the stressescan be reduced essentially in the ratio of the outer diameter of thetube.

Also, the alumina tube can be made with a thinner wall. The thick wallsin the beryllium oxide tube are for structural rigidity and also becausea large surface area is needed for water cooling. In the aluminum oxidetube the large outer diameter provides both structural rigidity and thearea available for water cooling. The calculated stresses in an aluminumoxide tube of 11/4 to 13/8 inches inside diameter and 11/2 to 15/8inches outside diameter amount, at the same power flow through the tube,to ten to fifteen percent of the breaking strength of the aluminumoxide. Thus the thin-walled aluminum oxide tube is completely comparableto the beryllium oxide tube in its ability to conduct away the heatgenerated in the tube without producing a stress which is a largefraction of the tensile strength of the tube.

However, the laser tube described in U.S. Pat. No. 3,501,714 has manyshortcomings both in design and fabrication. Many of these arise out ofthe use of a precision ceramic tube and high tolerance discs which arenot permanently bonded to the wall of the tube.

U.S. Pat. No. 3,501,714 describes a gas laser tube that uses expansionof tightly toleranced discs, both in surface finish and in diameter toinsure symmetric and uniform thermal coupling between the expandingdiscs and the ceramic tube wall. Besides making fabrication expensiveand difficult, if these tolerances are not met the expansion of theinner disc combined with the asymmetric heat flow through the outer wallof the tube can lead to large tangential stresses exceeding the tensilestrength of the ceramic tube. The tight tolerances that must be heldlimit the length of tube that may be machined. In the described laser,the discharge tube was less than 4" long even though the outer diameterof the tube was about 1.7".

Under symmetric radial heat flow conditions thermal stress in theceramic tube wall is generated due to the fact that the inner wall is ata higher temperature than the outer wall. Hence it expands moreproducing a tangential tensile stress in the outer wall. Under thecondition of symmetric radial heat flow the tangential tensile stress(and ceramic is weakest under tensile stress) in the outer wall isalready approximately 20% of the tensile breaking stress of the ceramic.Even if extremely tight tolerances are maintained it is estimated that acondition of asymmetric heat flow could result and the tensile stressesin the outer ceramic tube would be a factor of 5 greater than thestresses that would be encountered in a situation of symmetric radialheat flow. This condition could easily lead to breakage of the tube.

Furthermore, the surface finish tolerances quoted (outside of thediscs--16 RMS or better; surface finish on the inside wall of thealumina tube--32 RMS or better) limit the effective surface areaavailable for heat flow through the tube walls.

The laser described in U.S. Pat. No. 3,501,714 depends upon the use ofthick discs in order to maximize the surface area available for thermaltransfer between the gas discharge and the outer ceramic tube envelope.This fact, and the poor thermal contact between the inner discs and theouter ceramic envelope contribute to a high impedance to gas flow withinthe laser in several ways. First, it is necessary to use a cylindricalchannel through the thick discs for gas flow which creates a greaterimpedance to the gas flow. Secondly, the temperature of the discs ishigher resulting in greater gas flow impedance.

SUMMARY OF THE PRESENT INVENTION

It is therefore an object of the invention to provide an improved gaslaser.

Another object of the invention is to provide an improved method ofmaking a gaseous discharge tube, particularly suitable for use in an ionlaser.

Another object of the invention is to provide an ion laser having animproved gaseous discharge tube which is easily fabricated utilizing lowtolerance parts.

Another object of the invention is to provide a method of fabricating agaseous ion laser which is rugged and very reliable.

Another object of the invention is to provide a method of fabricating agaseous ion laser having a discharge tube which is very effective inconducting heat away from the discharge path.

In accordance with the present invention a gaseous laser is constructedwhich includes forming a discharge tube using a thin-walled,comparatively low tolerance ceramic tube. Heat-conducting members,preferably cup-shaped, are assembled and aligned in a spaced-apartrelationship within the ceramic tube. Preferably, cylindrical shields,which control gas pumping within the tube, are coaxially affixed to thecup members. Bore-defining apertures in the cup members or in discs madeof sputter-resistant material, are aligned with a mandrel under tension.While under tension the parts are permanently bonded within and to thetube.

With this construction heat generated by the plasma discharge isconducted rapidly through the cup members and through the ceramicenvelope. This rapid heat conduction is due to the intimate bond betweenthe cup-shaped members and the ceramic tube. More specifically, theinner members are first deformed to match the ceramic tube regardless ofroundness or surface finish irregularities in either part. Thesubsequent operation which, for example, can be brazing or soldering,provides a permanent metallic contact between the inner cup and theouter ceramic wall which leads to a high thermal conductivity betweenthese two parts.

As a result of these fabrication techniques tolerance requirements onthe inner and outer parts are drastically reduced. This in turn allowsquite long structures to be built without the necessity of machiningthese parts. Preferably the cup members which support the bore definingdiscs are made out of thin-walled copper. This material yields and doesnot stress the ceramic under thermal expansion. Therefore the tightcontact made by the brazing operation is achieved. It is not necessaryto leave a space for thermal expansiion as in the design of the laser isU.S. Pat. No. 3,501,714.

Fabricating the laser discharge tube with thin-walled cup members hasseveral advantages over the use of thick discs as in the laser of U.S.Pat. No. 3,501,714. The thin wall, as previously stated, allows easydeformation of the cup to match the inner diameter of the outer ceramictube without regard to either roundness or surface tolerances. This inturn provides a uniform thin capillary space which is then uniformlywetted by the brazed solder material which makes the thermal jointbetween the cup and the ceramic tube. In combination with a lowprecision ceramic tube, a laser tube is provided which is relativelyeasy to fabricate.

Another feature of the laser fabrication is the anode. The anode ismounted in the same manner as the other internal discharge tube members.Thus heat generated by the anode is transmitted directly through theceramic envelope by conduction. The anode is not in electrical contactwith cooling water, eliminating electrolysis. The resistivity andmineral content of the water, thus, do not affect tube life.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a dissected sectional view of an improved gaseous ion laser inaccordance with the present invention.

FIG. 2 is a cross sectional view of a cup-member and shield used in thelaser of FIG. 1.

FIG. 3 is a partial cross sectional view of the cup-member of FIG. 2shown within the ceramic tube during assembly.

FIG. 4 is a perspective view of the brazing shim shown in FIG. 3.

FIG. 5 is a plan view of an insertion/expansion tool used to assemblethe improved laser of the present invention.

FIG. 6 is a partial, cross-sectional view of a part of the tool of FIG.5.

FIG. 7 shows a cup member after insertion within the ceramic tube.

FIG. 8 shows an arrangement for fabricating the sputter resistant,bore-defining discs.

FIG. 9 is a perspective view of a disc prior to assembly.

FIG. 10 is a plan view of a brazing ring used to braze the disc of FIG.9 to a cup member.

FIG. 11 shows the brazing ring of FIG. 10 spot-welded to the disc.

FIG. 12 illustrates assembly of the cups and discs within the dischargetube.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a dissected sectional view of an improved gaseous ion laser 10in accordance with the present invention. Laser 10 includes a dischargetube 12, filled with an active gas such as argon or krypton, having anaxis 14 which is aligned with mirrors 16 and 18 (shown schematically)forming an optical cavity. A water jacket 20 (also shown schematically)surrounds the discharge tube 12. Water flows into the jacket at inlet22, absorbs heat conducted out of tube 12, and passes out of exit 24.Not shown is a solenoid, which surrounds the water jacket, whichconfines the plasma discharge in the well-known manner.

Discharge tube 12 includes a thin-walled ceramic tube 26, a cathodeassembly 28, an anode 30, and end window assemblies 32 and 34. Thelatter window assemblies are secured to nickel window support tubes 36to the cathode assembly 28 and anode 30 respectively.

The cathode assembly 28 includes two lead connectors, 38 and 40connected to a power supply (not shown) connected with a helical cathode42. Heat shields 44 are provided to prevent radiation or sputtering fromthe cathode 42 from entering into the region near window 32.

The plasma discharge path and cross section is determined by aperturesor bores 46 in a plurality of discs 48. Discs 48 are preferably made ofa sputter-resistant material. In FIG. 1 only seven discs are shown sincethe entire discharge tube 12 is not illustrated. In one actualembodiment more than 50 such discs are employed.

Discs 48 are bonded to at the periphery of openings 49 in thin-walledcup-shaped members 50. These are made of a material, such as copper,which is an excellent heat conductor and which is malleable. The rims 52of the cup members 50 are permanently bonded to the inside wall ofceramic tube 26. Gas return holes 54 are provided in the cup members 50.Heat generated by the plasma discharge is rapidly conducted through thediscs 48 and cup members 50 to and through the tube wall 26 where theheat is carried away by the water coolant.

Coaxially attached or formed with each of the cup members 50 is acylindrical ring or shield 56. These shields maintain the gas in regions58 at a comparatively low temperature. Each cylinder 56 is cooled bythermal conduction through cup 50 to tube wall 26. Without cylinder 56,the gas in region 58 is heated by thermal conduction from the discharge.If this happens a lesser number of gas atoms is stored within the tubeand a longer time is required after switch-on for the tube to reachequilibrium, since gas would have to move through the gas return holes54 to the anode and cathode regions before the proper operating pressurewas established in the region of the discharge. With cylinder 56 inplace, gas need only move from region 60 to region 58 to establishequilibrium operating conditions.

Gas pumping may take place by acceleration of the ions through the gasreturn holes 54 under the influence of the electric field in thevicinity of these holes. Cylinder 56 minimizes the number of ions inregion 58 by providing a surface for recombination 56 and only a smallchannel 62 through which ions may migrate into region 58.

It is well known to those skilled in the art that when a low pressuredischarge takes place in a continuous bore, such that all ions withinthe discharge are in freefall to the bore walls (theoretically describedas the Langmuir-Tonks Model), then gas pumping at low currents is towardthe anode and at higher currents toward the cathode. When the confiningstructure is in a region with no near walls, pumping is always towardthe cathode. Thus, by properly choosing the diameter of cylinder 56, itis possible, at the maximum discharge current for which the device isdesigned, to provide no net pumping. This is an important considerationin longer ion lasers in which it is difficult to provide sufficientbypass in an internal gas return of the normal type.

Since the direction of gas pumping by the discharge can be controlled bythe bore and shield 56 configuration and diameter, the tube can beconfigured to vary for different sections along the tube. Thus, forexample, one can have anode pumping in one region and cathode pumping inanother. This would then provide a pressure gradient within the tube inorder to minimize growth of plasma oscillations while still providing anoverall zero or small pressure differential.

If the gas return holes 54 are enlarged much beyond a diameter somewhatless than the bore diameter, the discharge may also travel through thegas return holes. If, however, the diameters and/or positions are variedthen the discharge would probably not pass through the bypass holes.This is because to do so would require a rather tortuous path throughthe structure. This tortuous path would also include sections where thedischarge was travelling transverse to the magnetic field rather thanparallel to it. This would tend to drive the discharge into the wall andraise the discharge potential per unit length.

Alumina tubes 26 are available from manufacturers such as "COORS" andfrom "McDANIELS" and are standard items having 1.50" O.D. and a 0.125"wall thickness. Tolerances on the inside cylinder are approximately±0.05" on the I.D. and 0.06" to 0.12" inch straightness. While it wouldbe straightforward to fabricate a bore tube as shown in FIG. 1 usingstandard self-jigging and auxiliary jigging techniques combined withserial brazing, the large tolerances on I.D. and straightness precludetheir use.

It is apparent that ceramic tube having small tolerances on insidediameter and straightness would be very expensive. The followingtechniques can be used to fabricate the ion laser bore tube to FIG. 1using available "standard toleranced" ceramic tubes.

The copper cup members 50 are formed from OFHC copper sheet by "drawing"as shown in FIG. 2. Afterwards the shield 56 is bonded, such as bybrazing, to the cup 50.

A brazing shim 64 (FIG. 4) is placed between the copper cup 50 andceramic tube 26 as shown in FIG. 3. One suitable brazing material"Ticusil." This material is a copper-silver eutectic with a smallpercentage of titanium added which has been used in the past for makingceramic to metal seals under what is known as the active metal process.In this process the titanium reduces the ceramic material to allow aceramic-to-metal seal in one operation. This obviates the necessity ofprior metalization of the ceramic through, for example, the well-knownmolymanganese or other processes.

It has been found desirable to provide a circumferentially extendinggrove 66 around the surface of the cup 50 rim. This grove accepts a lip68 formed in the brazing shim. The purpose of the lip/grove combinationis to prevent slippage of the shim during the brazing operation. Thisinsures a clean copper/ceramic contact at the very edge of the cup rim.

FIGS. 5-7 illustrate how the individual cup members 50 are expandedagainst the inside wall of ceramic tube 26 to form a secure mechanicalfit prior to brazing. The ceramic tube 26 is supported on a carriage 70which can be moved in a parallel fashion along rail 72. Within the tube26 is a floating mandrel 74. Fixed parallel to the rail 72 is anexpansion tool 76 which comprises an elongated portion 78 and anexpansion head 80. A copper cup 50 is first inserted over the expansionhead 80. Then carriage 70 is moved toward expansion tool 76 as indicatedby the dotted representation centering the cup 50 within the tube 26.

Suitable means, such as a hydraulic pump (not shown) drives a piston 79which compresses an elastomer ring (FIG. 6), otherwise confined,outwardly against a plurality of fingers 84 circumferentially locatedwithin the cup member 50. These fingers deform and force the rim 52firmly in contact with the ceramic tube 26. In this position (FIG. 7),it is ready for a later brazing operation.

The copper cups 50 are annealed prior to insertion within the tube 26 tominimize the force required to expand them and to allow them to bebetter forced into conformance with the ceramic tube 26. This isdesirable because of the slightly out-of-round inside diameter of thetube.

Plating of discs 48 prior to brazing is now described. First, tungstendiscs 48 are threaded onto a mandrel 86 spaced by metal spacers 88inside O-rings 90 (FIG. 8). This assembly is then nickel plated,0.0005"-0.0010" thick. The O-rings 90 mask the inside areas of the discsand the metal spacers 88 provide electrical contact to the discs.Masking prevents subsequent wetting of the surface by the brazingmaterial. A fabricated disc, having a nickel-coated outer area 92, isillustrated in FIG. 9.

After plating, the discs 48 are fired at 900° C. for 10 minutes inhydrogen. This anneals the nickel plating and improves thenickel-tungsten bond. A braze ring 94, as shown in FIG. 10, is thenformed from 0.030 dia Nicusil-3 wire, available from Western Gold andPlatinum Co. The braze ring is next spot welded at 96 to one face of atungsten disc 48 as shown in FIG. 11.

The tube assembly sequence is now explained. First, a cup 50 is placedon the expansion tool 76 and the tool expanded enough to hold the cup 50in place. The braze foil ring 64 is placed around the cup 50 and thecarriage/tube is moved until the cup is at the proper location insidethe tube 26. The tungsten mandrel 74 floats freely inside the tube 26and moves along with it on the carriage 70, slipping in and out of thehole in expansion tool. The cup 50 is then expanded, the pressure onelastomer 78 relieved, and then the carriage/tube moved of the expansiontool. A tungsten disc/braze ring assembly is then slipped over themandrel as shown in FIG. 12. The process is then repeated, alternatelyinstalling a cup 50 and a disc 48, until the bore is complete.

In actual practice the first and last tungsten discs 48 which match themandrel diameter can be previously brazed to their respective coppercups 50 using a higher temperature brazing alloy. These two discs thenestablish the bore centerline.

Prior to brazing, the tube/bore assembly with mandrel is placedvertically in a vacuum oven with end "B" (FIG. 12) upward. The mandrel74 is secured at one end and stretched taut by pulling on the oppositeend throughout the brazing cycle. This insures that all intermediatediscs 48 are aligned to form a straight bore after brazing.

A "transition" section, consisting of one or more sections havingprogressively larger disc hole 49 sizes, is provided at one or bothends. In this case the first and last "mandrel size" discs and alllarger size discs outward from these are pre-brazed.

As the assembly is heated the copper tends to expand more than theceramic because of the difference in their thermal expansioncoefficients. This results in the cups 50 being very tightly fitted inthe tube 26 when brazing temperature is reached. The copper becomesfully annealed at brazing temperature which is desirable so that it canyield to the strain set up by differential thermal expansion duringcooling and not pull away from the ceramic at the brazed joint.

The process of aligning these "floating" discs on a taut straightmandrel is very important to being able to fabricate very straight boreswithin low tolerance ceramic tubes. "Very straight bores" may be takenas those with overall straightness of less than 10% of the borediameter.

The cathode assembly 28 is encased in a copper enclosure 29 which mustform an air-tight seal with tube 26. The same expansion/brazingtechnique is used to form this connection. On the interior of the tube,the braze is required for good heat conductivity between the cup and thetube wall. On the ends, where a vacuum tight joint is also required,after expansion, the cup may be pressed into even more intimate contactwith the wall, with a chisel-shaped tool so that the cup will conform tothe same irregularities of the tube wall. This results in a bettervacuum seal.

The same fabrication technique is used to mount the anode 30. Theresulting configuration provides important advantages. The anode 30 ismounted by copper cups 51 similar in design to cups 50. As a result,heat generated by the anode is connected out of the tube through thecups 51. Thus, the anodes are cooled without any electrical contact withthe coolant water. The anode is electrically isolated from the rest ofthe tube avoiding the use of insulating varnish as in U.S. Pat. No.3,501,714, between the anode and the tube 26. Also, a lead wire (notshown) can be brought directly out of the tube without having to gothrough the tube 26 wall.

Another technique for providing high sputter-resistant material is tovapor deposit tungsten onto the central region of the copper cups 50 bywell known techniques. In this case, of course, there is not the sameflexibility of having the tungsten discs 48 slide around to centerthemselves on the mandrel. However a straight bore can still be achievedby using the insertion tool 76 with a small pin to hold the tungsten inthe center of the tube as determined by an external insertion machinereference. Then the copper cups are expanded to the walls of the tube sothat the copper cup rims conform to the tube wall while keeping thetungsten-plated center portion of the copper aligned.

Another technique for attaching the copper cups 50 to the alumina is asfollows. When one is expanded into contact with the alumina tube 26wall, which has been premetalized with Mo Mn or other techniques, thecopper cups can be pulse-soldered in place by means of a pulsedinduction heating apparatus which heats the cups 50 and thecircumferential metalization of the ceramic and thus melts the solder.The cooling of the alumina wall after the pulses terminate cools thesolder below its melting point so that the tool can be withdrawnimmediately with the copper cup soldered in place. This technique isdesirable if permanent magnets are used inside a tube, since it is amethod of attaching the copper cups at low temperature where the magnetdoes not de-magnetize.

Since the tube operates at relatively low temperature, the interuptionof water cooling does not result in boiling of the water remaining inthe cooling jacket. Calculation shows that the stored heat in the tubeis insufficient to even raise the bore tube 26 to the boiling point ofwater. Air-cooled tubes are also feasible with this laser design.Cooling fins are attached directly to the external surface of thealumina tube 26.

In one actual embodiment, the following dimensions were used:

    ______________________________________                                        cup 50         thickness    .030"                                                            dia. of opening 49                                                                         .31"                                                             O.D.         1.125"                                            brazing ring 64                                                                              thickness    .002"                                             disc 48        bore 46 dia. .110"                                                            O.D.         .50"                                                             thickness    .010"                                             shield 56      O.D.         3/4"                                                             I.D.         5/8"                                              ______________________________________                                    

What is claimed is:
 1. A method of fabricating a gaseous laser dischargetube comprising:assemblying a plurality of spaced-apart heat-conductingmembers, each having a discharge defining central aperture generallyaligned with the tube axis, within and in contact with anelectrically-insulating tube; tensioning a mandrel provided through thecentral apertures to bring the central apertures into exact alignment;and permanently securing the heat-conducting members to theelectrically-insulating tube.
 2. The method of claim 1 wherein saidsecuring step comprises brazing the heat-conducting members to theelectrically-insulating tube.
 3. The method of claim 2 wherein theassembly step includes the additional step of inserting brazing materialbetween the heat-conducting members and the electrically-insulating tubeprior to brazing.
 4. The method of claim 3 wherein the brazing step isaccomplished by baking the entire tube assembly while the mandrel is intension.
 5. The method of claim 4 wherein the baking step occurs withthe mandrel vertically oriented.
 6. The method of claim 2 wherein theassembly step includes the steps of:(a) forming cup-shaped members of aheat-conducting material; (b) inserting the cup-shaped membersspaced-apart within the tube; (c) inserting brazing material between thecup-shaped members and the tube wall; and (d) expanding the cup rim intothe interior wall of the tube to hold the cup-shaped members in place.7. The method of claim 6 including the additional step of insertingdiscs of heat-resistant material having a bore-defining central aperturetherein, within and in contact with each of the cup-shaped members. 8.The method of claim 6 wherein a brazing material is provided between thediscs and the cup-shaped members.
 9. The method of claim 8 wherein atleast two cup-shaped members have discs bonded therein before insertingthem within the tube to define a bore center line.
 10. The method ofclaim 6 wherein the assembly step additionally includes forming acylindrical shield ring coaxially as part of each of said cup-shapedmembers.
 11. The method of claim 2 wherein the heat-conducting membersare annealed before assemblying within the tube.
 12. The method of claim1 including the step of terminating one end of the laser tube with acathode assembly and the other end with an anode.
 13. The method ofclaim 12 wherein said anode is affixed within the tube by attaching itto the tube wall by additional heat-conducting members.
 14. The methodof claim 1 including the step of surrounding each of the centralapertures with a sputter-resistant material.